For example, since a metal can, which is one form of packaging container for food and beverage, has many advantages in that long-term preservation of the contents is possible because it is superior in mechanical strength, packing the contents at a high temperature and sealing as is, as well as easy sterilization treatment such as retort treatment etc. are available, which leads to high reliability in safety and hygiene as a packaging container, and further, preservation of the contents in a heated state is possible and separate collection of can after use is relatively easy. Thus, various contents have been packed therein and metal cans have been used in a large quantity in recent years.
Conventionally, paints containing a thermosetting resin as a main component have been applied to metal cans, because the inner and outer surfaces of a metal can for food and beverage should maintain flavor of the contents and prevent corrosion of the metal can, or for the purpose of improving good appearance of the outer surface of the can and protecting printed surfaces. However, since such metal cans require use of a large amount of solvents during production, they pose problems of an influence on the environment due to de-solventing during production, problems in hygienic aspects due to residual solvent in the coating, degraded flavor property due to residual oligomer produced by defective reaction during thermosetting and the like.
To overcome these problems, it has been proposed to laminate a plastic film on a metal, and, since more superior aspects have been found in terms of adhesive power, heat resistance, mechanical strength, flavor property, processing suitability and the like than other resins, of the thermoplastic resins, a polyester film is more often laminated on a metal sheet. As a metal can made by processing such a film-laminated metal sheet, what is called a 3-piece can (hereinafter to be abbreviated as a 3P can) and a 2-piece can (hereinafter to be abbreviated as a 2P can) have been proposed. Of these, prevailing of 2P cans has been desired from the aspect of seamless processing.
As a general production method of 2P can, a typical method comprises producing a seamless can by laminating a plastic film, punching a laminated metal sheet with a can making machine and subjecting the sheet to a drawing and ironing step. In such a can making step, a film is required to have formability to follow spreading of a metal sheet while receiving a shear of drawing and ironing, without being subject to crack and detachment from a metal sheet, and is also required to be free of whitening of film due to heating in a can making step, and the like.
In response to such required characteristics, a polyester film containing a polyethylene terephthalate polyester resin having a particular intrinsic viscosity and a polybutylene terephthalate polyester resin having a particular intrinsic viscosity has been proposed.
For example, in U.S. Pat. Nos. 2,882,985, 3,020,731 and the like, by optimizing semi crystallization temperature and semi crystallization time of the polyethylene terephthalate polyester resin and the polybutylene terephthalate polyester resin to be added, the followability a film to deformation of metal and retort whitening are suppressed.
In JP-A-10-195210, JP-A-10-110046 and the like, moreover, followability of a film to deformation of metal and flavor property by heat treatment (crystallization treatment) after lamination are improved by optimizing thermal property and planar orientation of a polyethylene terephthalate polyester resin and a polybutylene terephthalate polyester resin to be added. In so doing, when transesterification has proceeded too much between two kinds of polyesters, the crystallinity of the film cannot be increased. Therefore, for example, melting time of resin during film forming and the amount of heat applied to a film during subsequent stretching and heat treatment step of the film are designed to be decreased.
Moreover, JP-A-2002-179892, JP-A-2002-321277 and the like propose a polyester film which, though it is a blend film of a polyethylene terephthalate (PET) polyester resin and a polybutylene terephthalate (PBT) polyester resin, is alleged to be able to maintain heat seal property and formability because PET phase and PBT phase comprise independent crystals. This film is considered to be usable for forming transfer, forming container, metal adhering and the like, and forming a film by extruding a blended resin by a vent-type extruder to give a film has been proposed to make PET phase and PBT phase independent crystals.
However, according to the consideration of the present inventors, it has been found that, in conventional blend type polyester films proposed above, when a film is melted at a near melting point or not lower than the melting point, adhered to a metal and the like and then cooled, problems occur in that the film is whitened and the design of the film is degraded. In addition, a problem has been found that the film on a metal sheet is damaged easily.